Dark theme doesn't seem to affect Vivaldi

Summary

This is the theme that I'm using. I think it's the default, because I don't remember choosing it, but it's what I would have chosen regardless. My eyes are rather ensitive to bright light.

The theme apparently applies to the applications that are part of the Xfce DE, but not to one that I installed from the repository--in particular, Vivaldi. Opening any of the many sites that it hosts provides a painful blast of white light.

Steps to reproduce

Just launch Vivaldi (on hardware) and you should notice right away that it is not using any kind of dark theme.

Expected result

I expected web pages to be displayed in a dark theme, as they are on a fully-updated Xfce RC2 VM that runs on my daily-driver laptop. (See similar image in comment below.)

Actual result

This should be dark as well (Xfce on bare metal) but it's apparently unaffected by the theme. I have no idea why the hardware version differs from the virtual version, but as I pointed out, it's painful to look at.

Environment

• Is system up to date?

Repo

Shannon (stable)

Desktop Environment

Xfce

System details

Other comments

No response